The greatly increased use of computers in offices and manufacturing facilities for data, imaging and video transmission, has given rise to increased demands upon the signal transmitting cable used to interconnect the various electronic peripheral devices with, for example, computers. These demands must be met in order to insure substantially error free signal transmission at high bit rates. In addition, inasmuch as such cables are generally used within a building, the cable must be fire resistant and/or flame retardant.
The danger of the spread of fire is compounded in those cases where the cable extends from floor to floor, in which case it is referred to as a riser cable. This cable is often extended upward or downward for more than two stories, therefore, Underwriters Laboratories performs stringent tests to verify that the cable will perform satisfactorily. This includes a burn test (UL-1666) in order to establish a CMR rating for communications cable used in riser and general purpose applications.
The UL Test 1666, known as a vertical tray test is used by Underwriters Laboratories to determine whether a cable is acceptable as a riser cable. In that test, a sample of cable is extended upward from a first floor along a ladder arrangement having spaced rungs. A test flame producing approximately 527,500 Btu per hour, fueled by propane at a flow rate of approximately 211.+-.11 standard cubic feet per hour, is applied to the cable for approximately thirty minutes. The maximum continuous damage height to the cable is then measured. If the damage height to the cable does not equal or exceed twelve feet, the cable is given a CMR rating approval for use as a riser cable.
There are, in the prior art, numerous cables which perform satisfactorily in a riser application, meeting both the electrical requirements and the flame spread requirement. In U.S. Pat. No. 4,284,842 of Arroyo et al., there is shown one such cable in which the multi-conductor core is enclosed in an inorganic sheath which is, in turn, enclosed in a metallic sleeve. The metallic sleeve is surrounded by dual layers of polyimide tape. The inorganic sheath resists heat transfer into the core, and the metallic sheath reflects radiant heat. Such a cable effectively resists fire and produces low smoke emission, but requires three layers of jacketing material. Another example of a multilayer jacket is shown in U.S. Pat. No. 4,605,818 of Arroyo. In U.S. Pat. No. 5,074,640 of Hardin et al., there is disclosed a cable for use in plenums or riser shafts, in which the individual conductors are insulated by a non-halogenated plastic composition which includes a polyetherimide constituent and an additive system. The jacket includes a siloxane/polyimide copolymer constituent blended with a polyetherimide constituent and an additive system, including a flame retardant system. In U.S. Pat. No. 4,412,094 of Dougherty et al., a riser cable is disclosed wherein each of the conductors is surrounded by two layers of insulation. The inner layer is a polyolefin plastic material expanded to a predetermined percentage, and the outer layer comprises a relatively fire retardant material. The core is enclosed in a metallic jacket and a fire resistant material. Such a cable also meets the requirements for fire resistance and low smoke. However, the metallic jacket represents an added cost element in the production of the cable. In U.S. Pat. No. 5,162,609 of Adriaenssens et al., there is shown a fire resistant cable in which the metallic jacket member is eliminated. In that cable, each conductor of the several pairs of conductors has a metallic, i.e., copper center member surrounded by an insulating layer of solid, low density polyethylene which is, in turn, surrounded by a flame resistant polyethylene material. The core, i.e., all of the insulated conductors, is surrounded by a jacket of flame retardant polyethylene. Such a structure meets the criteria for use in buildings and is, apparently, widely used.
As the use of computers has increased, and more particularly, as the interconnections of computers to each other, and to telephone lines, has mushroomed, a cable for interior use should, desirably, provide substantially error free transmission at very high frequencies. The satisfactory achievement of such transmission has not been fully realized because of a problem with most twisted pair and coaxial cables which, while not serious at low transmission frequencies, becomes acute at the high frequencies associated with transmission at high bit rates. This problem is identified and known as structural return loss (SRL), which is defined as signal attenuation resulting from periodic variations in impedance along the cable. SRL is affected by the structure of the cable and the various cable components, which cause signal reflections. Such signal reflections can cause transmitted or received signal loss, fluctuations with frequency of the received signals, distortion of transmitted or received pulses, increased noise at carrier frequencies and, to some extent, will place an upper signal frequency limit on twisted pair cables. Some of the structural detects that cause SRL are conductors which fluctuate in diameter along their length, or where, for whatever reason, the surface of the wire is rough or uneven. Insulation roughness or irregularities, excessive eccentricity, as well as variations in insulation diameter, may likewise increase SRL. With dual insulated conductors, as shown in the aforementioned Dougherty et al., and Adriaenssens et al., patents, the problem of achieving uniformity of insulation is compounded because of the difficulty of forming a first layer that is substantially uniform and then forming a second, substantially uniform layer over the first. If the first layer is soft or compressible, the second layer can distort it, thereby increasing SRI, to an undesirable level. If, in turn, the second layer is compressible, it can be distorted by the helical member used to bundle the cable pairs, or during the twisting process. Should the conductors of a twisted pair have varying spacing along their length, SRL can be undesirably increased. The presence of metallic shielding members or sleeves can also lead to undesirable increases in SRL.
For a Category V cable, which is the highest category, i.e., the category wherein the cable is capable of handling signals up to 100 MHz, the cable must meet the UL designated EIA/TIA 568 standard rating Proposal 2840 which involves attenuation, impedance, cross-talk, and SRL. For a Category V cable, the SRL, in dB, should be, at 20 MHz, 23 dB or more. For frequencies above 20 MHz, the allowable SRL is determined by ##EQU1## where SRL.sub.200 is the SRL at 20 MHz and f is the frequency. It should be understood that the measured SRL is given by dB below signal and hence, in actuality, is a negative figure.
The difference between the required or allowable SRL and the measured SRL is known as SRL margin. Therefore, the greater the SRL margin of a cable, the better the performance thereof. It can thus be appreciated that the necessity for flame retardance or fire resistance, especially in riser cables, and the desirable end of minimizing SRL, resulting in unimpaired signal transmission, are not amenable to a simple solution. The achievement of a high level of flame retardance by the prior art methods as noted in the foregoing can, and most often does, lead to increased SRL, as does the presence of metallic sleeves or the like. While it is by no means impossible to achieve good SRL characteristics with some of the prior art flame retardant riser cables, the cost involved in assuring uniformity of the various conductors and double insulation layers, while not prohibitive, can be substantially more than is economically feasible.